You can hardly call him terrible if he keeps winning games. Sure his mechanics aren't what the NFL looks for in a QB but he gets the job done. I think he could also be pretty successful in a check down attack. He's not going to be accurate at the deep ball but then he hasn't even been a starter for a full year yet. It's not impossible that he develops into a good NFL-style passer, even if the deck is stacked against him.
Fox is just bitter that Tebow is actually winning, I'm sure they decided to throw him in there to show everyone that he was right and they can't win with him, and it backfired. No Luck for the Broncos now, they might even make the playoffs still.